Mil-spec tube will screw in to any lower. The main difference is the diameter of the tube after the threads. Commercial tubes are a bit bigger as the above pic shows and the end of the tube us has an angled end instead of a flat end like on a mil-spec tube.

I don't know where you guys got your information from, but you need to check your sources again....

Commercial tubes DO NOT have the same threading as Mil-spec. The thread count and pitch are the same, but the depth and height of the threads is different. This is why a commercial tube will wiggle in the lower until it is tightened by the nut.

On a mil-spec tube the threads are 100% exact.

Now they could have fixed that fuck up since I last bought a commercial tube, but that is why they were cheaper and I think its still the reason they are cheaper.